---
title: "MVCC (Multiversion Concurrency Control)"
slug: "mvcc"
category: "glossary"
tags: ["glossary", "database", "postgres", "mvcc", "concurrency", "transactions"]
status: "stable"
last_updated: 2026-10-01
summary: "MVCC keeps multiple row versions so each statement reads a consistent snapshot; in PostgreSQL reads never block writes and dead row versions need vacuuming."
related: ["[[glossary/transaction-isolation]]", "[[glossary/vacuum]]", "[[glossary/write-amplification]]", "[[glossary/optimistic-locking]]", "[[backend/postgres]]"]
---

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## Overview

Multiversion concurrency control gives each statement a snapshot of the data instead of making readers wait for writers. In PostgreSQL's words, "reading never blocks writing and writing never blocks reading". The price is that an `UPDATE` or `DELETE` leaves the old row version behind, so [[glossary/vacuum]] must reclaim it and long-running transactions cause bloat.

## Rules

- Snapshot scope follows [[glossary/transaction-isolation]]: Read Committed takes a new snapshot per statement; Repeatable Read and Serializable keep one for the whole transaction.
- Writers still block writers on the same row. Use row locks or [[glossary/optimistic-locking]] for read-modify-write conflicts.
- Every update writes a new row version, and new index entries unless it is a HOT update ([[glossary/write-amplification]]).
- Old versions live until no snapshot can see them, so an idle-in-transaction session blocks cleanup. Keep transactions short.
- Implementations differ: PostgreSQL keeps old versions in the table, while MySQL InnoDB keeps them in undo logs.

```sql
SELECT xmin, xmax, * FROM accounts WHERE id = 1;  -- row-version metadata
```

## Related

- [[glossary/transaction-isolation]]: which snapshot a transaction sees.
- [[glossary/vacuum]]: reclaims dead row versions.
- [[glossary/write-amplification]]: the write cost of new versions.
- [[glossary/optimistic-locking]]: handles writer-writer conflicts.
- [[backend/postgres]]: concurrency and locking.